Advantages of high efficiency and high torque at low speed are realized by employing rapid near-top-dead-center injection of fuel jets. Such direct injection creates its own turbulence and burns at a characteristically turbulence-limited combustion rate.
A characteristic disadvantage of the modern conventional diesel engine is the tendency to produce particulate matter, particularly soot, as a result of incomplete oxidation of the fuel. Another tendency is to produce excessive nitrogen oxides (NOx) due to the fact that diffusion. combustion is characteristic of such systems-combustion takes place around the stoichiometric or maximum temperature. It has long been known that measures which tend to reduce the production of nitrogen oxides also tend to increase the production of soot and vice versa. However, if soot production could be inhibited, the production of nitrogen oxides could also be reduced. Moreover, soot reduction should also increase power density.
A related issue that impacts on soot production relates to a strategy used to reduce nitrogen oxides. This strategy is partial re-circulation of exhaust gas (EGR). EGR helps to reduce combustion temperature and, as a result, NOx. The reduction of combustion temperature however impacts the rate of oxidation of soot formed as a combustion intermediate. Further, it has been found that it also tends to reduce burning rate. Such incomplete combustion results in reduced efficiency of the engine overall. Therefore, in light of the fact that soot production and incomplete combustion limit the extent to which exhaust gas re-circulation (EGR) can be utilized, the advantage of a means for increasing the combustion rate and the soot oxidation rate is apparent.
Developments in combustion engine technology have shown that compression ignition engines can be fuelled by gaseous fuels instead of diesel fuel. Some of these developments allow performance and/or efficiency to be maintained with gaseous fuels. Examples of such fuels include natural gas, methane, propane, ethane, gaseous combustible hydrocarbon derivatives and hydrogen. Substituting diesel with such gaseous fuels results in emissions benefits over diesel. Specifically, lower NOx and soot production are found in the exhaust gas created in such engines.
A method used to ensure that gaseous fuels match, for the most part, the performance and efficiency found in diesel-fueled ignition engines, relies on diffusion combustion. That is, gaseous fuel is directly injected at high pressure into a combustion chamber where an ignition source is usually used to ignite the gaseous fuel. Due to such direct injection and diffusion combustion, this fuel generally suffers from the same issues noted above in regards to soot and NOx generation, albeit at significantly lower levels than is the case with diesel fuel. The same zone of incomplete oxidation found in regards to combustion resulting from diesel-fuelled compression ignition strategies is thought to result. As such, while gaseous fuels provide a significant reduction of particulates and NOx, these fuels, directly injected, are governed by some of the same physical processes found in diesel-fuelled compression ignition engines. Therefore, room is available to manage soot and particulate production in both gaseous-fuelled and diesel-fuelled direct injection engines.
Dec, J. E., “A Conceptual Model of DI Diesel Combustion based on Laser-Sheet Imaging”, SAE 970873, 1997, provided a physical understanding of conventional diesel fuel combustion for the quasi-steady period of burning. It appears that combustion takes place in two phases. The first occurs in the rich mixture created by entrainment of air into the fuel jet. Here the equivalence ratio is so high that the flame temperature is low (perhaps around 1600° K.) and soot forms by pyrolysis due to the shortage of oxygen. A soot-rich zone is created that is surrounded by a thin, region in which final mixing and any remaining chemical reactions occur. Understanding the behavior of this soot rich zone provides a starting point for reducing the production of soot in the diesel and natural gas engines described above.
Sjoeberg, in “The Rotating Injector as a Tool for Exploring DI Diesel Combustion and Emissions Formation Processes”, ISSN1400-1179, 2001, provided a rotating injector that, in effect, caused turbulence that impacted on the soot-rich zone by moving the fuel jet throughout the combustion chamber. Such a strategy, however, is difficult to implement. A rotating injector introduces moving parts to the engine that are susceptible to wear and durability issues.
U.S. Pat. No. 5,862,788 discloses a reaction chamber within the piston head that discharges a mixture of fuel and air towards the center of the piston bowl. Here, the chamber is designed so that fuel is admitted and circulated in the chamber and heated before being directed at a soot cloud thought to form within the piston bowl. The requirement to mix fuel within the chamber limits the effect of the fuel/air reaction products as the soot cloud is tied to the fuel jets. As the piston is, in general, moving away from top dead center during combustion of the fuel, the impact on the soot-rich zone is limited. Further, some fuel needs to be directed into the reaction chamber according to this invention, limiting placement of the orifice leading to the chamber.
The present invention deals with the above noted problems related to directly injected fuels used in internal combustion engines.